I need a small motor that will have a fixed, stable RPM. Size and voltage are not critical. Torque should be at least 50 oz-in or so, and RPM can be anything in the range 20-300; I can adjust by gearing. But I’d like it to be as simple as possible, minimal complexity electronics and very stable RPM under varying load. Any thoughts? Thanks!

What kind of variance would be acceptable for your application? To have a DC motor run at a steady speed with a varying load will require some kind of feedback. I am not sure what you consider simple or with minimal complexity, but you could use something like our Jrk Motor Controllers with Feedback to do closed-loop speed control with tachometer feedback (e.g. using one channel of a quadrature encoder). I recommend reading through the jrk controller’s user’s guide to get an understanding of how to do this and see if it would work for you. Additionally, my post here (as well as the one I link to from there) might also be helpful for understanding how the feedback works and how to appropriately configure the jrk for your system.

If you can give me more specific requirements, I might be able to suggest a suitable gearmotor. For example, you mentioned a load, but then said it will vary; what is the full range you expect? What RPM do you want the motor to run at (without additional gearing)? Also, do you have a preference for operating voltage?

Brandon - Thank you for the reply! I want to build a novelty clock with oddly shaped 3D-printed gears, moving mechanical figures, and so forth. It will sit on a small support box which contains a hidden motor driving the whole mechanism from below. I’d love for it to be able to run on batteries, but I suspect that I’ll have to power it with a wall-wart transformer. Because it’s a clock, the motor speed must be fixed. But because I will be designing the gearing myself, I can be flexible about the actual motor speed and adjust the gear ratios as needed. Also, I can get whatever power supply is needed to power the motor, so voltage is not critical. I skimmed the JRK Motor Controller info you supplied, but it looks like an attached computer is needed. This, of course, is impossible in my application. Your thoughts? Thanks!

Thank you for the additional information. Since your application is a clock and will likely need to run nonstop for very long periods of time and need to stay very constant, a brushed DC motor is probably not the best solution. You might consider something like a stepper motor instead. You could use one of our current limiting stepper motor drivers, which accept a two-pin step and direction interface. Since it sounds like you are not likely to need to change directions, this could be reduced to just a step signals by tying the DIR pin high or low. This step signal could be sent from a separate microcontroller like an Arduino or some other device that can output a square-wave with the frequency you need.

By the way, just to clarify for anyone else reading this, while a PC is necessary to configure the jrk controllers, the jrk does not need to be connected via USB to a computer to function; it can be controlled from an analog voltage input, TTL serial commands, or a hobby RC signal as well.

Brandon - Thanks for the update. I really want to avoid microcontrollers or any other complexity, at least for now. I was hoping that there was such a thing as a self-contained motor with fixed rotation speed. I know that very lower power versions of them exist, given the multitude of wall clocks that operate on a single AA cell. I was hoping that a higher power version of the same thing existed. Oh well. I’m tracking down some such motors that run on AC and sync to the 60 Hz power line, so I may end up going in that direction. Thanks for your help.